This invention relates to a method and apparatus for producing calcium phosphates from phosphoric acid and mineral calcium compounds. More particularly, the invention relates to a method for manufacturing animal feed grade phosphates, mono-calcium phosphate (MCP) and di-calcium phosphate (DCP) or a mixture thereof containing P and Ca in a specified ratio and in a form in which the phosphorus component can be readily absorbed by the animal organism. Such mineral feed additives must comply with certain standards of purity with respect to content of fluorine, arsenic and heavy metals. To facilitate handling and use it is important that these feed grade phosphates are in the form of strong and free-flowing granulates of appropriate liter weight and grain size, and that they possess good storage properties.
The manufacture of such animal feed grade phosphates by reaction between phorphoric acid and fine-grained mineral calcium compounds is known.
It is further known to carry out the reaction by direct conversion between a purified, comparatively concentrated phosphoric acid and a purified calcium component which may, if necessary, be in the form of an aqueous suspension. During conversion the reaction mixture will first pass through a glutinous and sticky phase, after which it will harden as reaction continues. This gives consistency and handling problems which cause serious trouble during these stages of the reaction.
When the calcium components are present in the form of CaCO.sub.3 considerable quantities of gas will be formed during the reaction, which further increases the consistency and handling problems in the apparatus. This is probably the reason why so many prior known processes for the manufacture of animal feed grade phosphates prescribe the use of other calcium compounds which cause less serious consistency and handling problems.
The reaction mixture is so glutinous and viscous that very powerful mixing is necessary during further processing. In the prior known granulating techniques there are difficulties in handling the mixture without recycling considerable quantities of the reacted product.
The consistency problems also make it difficult to achieve homogenization and proper mixing of the reaction mass. Local concentrations of acid resulting from inadequate mixing will in turn result in a product with inferior handling and storage properties.
Because of these special difficulties no one, as far as is know, has yet succeeded in developing a simple and technically reliable process for the manufacture of non-dusting granulated animal feed grade phosphates from phosphoric acid and CaCO.sub.3. According to South African Pat. No. 66/7774, which relates to such a reaction process, the granulating stage is eliminated and the viscous and sticky reaction mixture is transferred instead to a slow-moving conveyor belt on which the reaction is completed during hardening of the reaction product. The solid mass is then crushed by means of a rotary crusher. However, this results in an unsatisfactory product with angular, uneven grains and considerable dust.
According to Norwegian Pat. No. 100,875 the reaction is carried out in the form of a batch process without an aqueous phase, and with rapid mixing with finely ground limestone and 80% phosphoric acid in a pan mixer. Agglomerates appear for a short period but these are broken down by the mixer as they appear and a slightly plastic powder is formed. A particular disadvantage in this process is the time factor, since the reaction requires a period of up to fifty hours. Neither is the powdered product sufficiently free-flowing, nor is it particularly suitable for mixing with the usual animal feedstuffs.
However, it is also a known prior procedure to reduce the reaction period by utilizing a greater amount of water and using a known prior wet granulation technique. The aqueous reaction components must then be distributed in a greater quantity of reacted return material, recycled from the product screens to the granulator. Swedish Patent Application Ser. No. 340,443, open to public inspection, relates to such a process, but its workability depends on the use of considerable quantities of reacted, recycled material, as much as up to twenty-five times the weight of the reaction mixture, during the granulating process. The reaction components are distributed in this recycled material. The use of such recycled material complicates the process and reduces the production capacity of the granulation stage. Close and careful control of the reaction conditions with respect to the water/acid ratio, reaction time and temperature is also difficult with this process.